Tính

a) -4/7 - 5/14 × -39/25 - 1/42 : (-5/6)
= -4/7 - 5/14 × -39/25 - 1/42 × (-6/5)
= -4/7 + 5/14 × 39/25 - 1/42 × (-6/5)
= -4/7 + 5/14 × 39/25 + 1/42 × 6/5
= -4/7 + 5/14 × 3/5 + 1/42 × 6/5
= -4/7 + 15/70 + 6/210
= -4/7 + 3/14 + 1/35
= -8/14 + 3/14 + 2/70
= -5/14 + 2/70
= -25/70 + 2/70
= -23/70

b) (4/5 + -9/7) × 2025/2030 + (-5/7 - -6/5) × 2025/2030
= (4/5 - 9/7) × 2025/2030 + (-5/7 + 6/5) × 2025/2030
= (4/5 × 7/7 - 9/7 × 5/5) × 2025/2030 + (-5/7 × 5/5 + 6/5 × 7/7) × 2025/2030
= (28/35 - 45/35) × 2025/2030 + (-25/35 + 42/35) × 2025/2030
= (-17/35) × 2025/2030 + (17/35) × 2025/2030
= (-17/35 + 17/35) × 2025/2030
= 0 × 2025/2030
= 0

Bài 2:
a) 1/3x - 2/5 = -7/15
LCM của 3, 5, 15 là 15
Nhân cả phương trình với 15 để loại bỏ mẫu số
15/3x - 15/5 = -7/15 × 15
5x - 6 = -7
5x = -7 + 6
5x = -1
x = -1/5

b) 2x - 3 × 3 = 2x + 92 - 0
2x - 9 = 2x + 92
-9 - 92 = 2x - 2x
-101 = 0
Phương trình vô nghiệm.

c) 0.2 - x = -1 2/5
0.2 - x = -7/5
-x = -7/5 - 0.2
-x = -7/5 - 1/5
-x = -8/5
x = 8/5

d) x + 3/2 = -5/12
x = -5/12 - 3/2
x = -5/12 - 18/12
x = -23/12

e) 3 × (-2x + 5/2) × (x + 11)
= 3 × (-2x × x + 5/2 × x + 11 × -2x + 5/2 × 11)
= 3 × (-2x^2 + 5/2x - 22x + 55/2)
= -6x^2 + 15/2x - 66x + 165/2